Method for producing and testing a corrosion-resistant channel in a silicon device

ABSTRACT

A method for producing a corrosion-resistant channel in a wetted path of a silicon device enables such device to be used with corrosive compounds, such as fluorine. A wetted path of a MEMS device is coated with either (1) an organic compound resistant to attack by atomic fluorine or (2) a material capable of being passivated by atomic fluorine. The device is then exposed to a gas that decomposes into active fluorine compounds when activated by a plasma discharge. One example of such a gas is CF 4 , an inert gas that is easier and safer to work with than volatile gases like ClF 3 . The gas will passivate the material (if applicable) and corrode any exposed silicon. The device is tested in such a manner that any unacceptable corrosion of the wetted path will cause the device to fail. If the device operates properly, the wetted path is deemed to be resistant to corrosion by fluorine or other corrosive compounds, as applicable.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates generally to silicon devices (includingMEMS devices) and more specifically to a method for producing andtesting a corrosion-resistant channel in a silicon device.

[0003] 2. Description of the Background Art

[0004] A relatively recent development in the semiconductor industry isto use microelectromechanical systems (MEMS) in semiconductor andpharmaceutical manufacturing processes. MEMS devices are typicallysilicon chips that include miniaturized mechanical components, such asactuators, mirrors, levers, diaphragms, or sensors. MEMS devices mayalso include electronic circuitry.

[0005] When MEMS devices are employed in semiconductor andpharmaceutical manufacturing processes, they are exposed to the chemicaland biochemical substances used in such processes. The part of the MEMSdevice exposed to fluids (i.e., gases or liquids) during operation iscommonly referred to as the “wetted path.” The wetted path may bedifferent from the primary flow path (i.e., the path along which thefluid is intended to travel) because fluids sometimes can enter intoopen spaces other than the primary flow path, referred to as the “deadvolume”

[0006] The materials of the MEMS device that form the wetted path mustbe able to withstand corrosion or attack from fluids flowing through thedevice. In applications where corrosive fluids are present, thematerials in the wetted path are critical, and compatibility of all thematerials present is essential. In products requiring high purity, suchas those used in the semiconductor or pharmaceutical industries, even asmall amount of corrosion is unacceptable.

[0007] In many MEMS devices the wetted path is formed from a siliconchannel, as MEMS devices are usually comprised at least in part bysilicon wafers. The microvalve illustrated in FIG. 1(a) and (b), in“off” and “on” states, respectively, is an example of a MEMS device withsilicon in the wetted path. The valve is used to finely control the flowof fluids. The microvalve includes a heater plate 22, a diaphragm plate28, and a channel plate 30.

[0008] The channel plate 30, which is formed from a silicon wafer,includes an input port 32 and an output port 34. The illustrated valveis a normally-open valve in that fluid entering input port 32 normallyis able to travel freely through the valve 100 and out via output port34, as depicted in FIG. 1(a). An example of a normally-closed valve isdescribed in U.S. Pat. No. 6,149,123 (the “'123 patent”), the contentsof which are incorporated by reference as if fully disclosed herein.

[0009] The diaphragm plate 28 includes a cavity 41 which holds athermopneumatic liquid. The thermopneumatic liquid also extends upthrough channels 56 in the heater plate 22. When control circuitry (notshown) associated with the valve indicate the valve should close, theheater plate 22 warms the thermopneumatic liquid. The diaphragm plate28, which is formed from a silicon wafer, includes a flexible diaphragm44. When the thermopneumatic liquid is heated, it expands, causing thediaphragm 44 to bend and block input port 32. As illustrated in FIG.1(b), when the input port 32 is blocked, the valve is closed and anyfluid flow is severely restricted (e.g., less than 1 sccm).

[0010] The wetted path of valve 100 is cavity 43, the input and outputports 32, 34, and any exposed surfaces around the foregoing, all ofwhich are formed from channel plate 30 and diaphragm plate 28. As thesetwo plates 28, 30 are made of silicon wafers, the wetted path is asilicon channel.

[0011] A valve similar in operation to valve 100 is described in U.S.Pat. No. 4,996,646 (the “'646 patent”). Another example of anormally-open valve is described in U.S. Pat. No. 6,129,331 (the “'331patent”). The contents of the '646 patent and the '331 patent areincorporated by reference as if fully disclosed herein.

[0012] As stated above, the fluids flowing through MEMS devices, such asthe valve illustrated in FIGS. 1(a) and (b), must not corrode thedevice. For instance, if fluids were to sufficiently corrode the valveof FIG. 1(a) and (b), the diaphragm 44, which is made up of a thin layerof silicon, would eventually break under operation. In addition, thecleanliness of the semiconductor or pharmaceutical process may becompromised by the products of the reaction of such fluids with thesilicon. While silicon is non-reactive with most process gases andsingle constituent acids, it reacts with atomic fluorine, F, and othercompounds which can spontaneously dissociate to atomic fluorine. Asilicon atom, Si, will react with fluorine atoms to form SiF₄, avolatile component which vaporizes off the surface, thereby corrodingthe silicon. Consequently, there is a need to protect the wetted pathfrom fluorine.

[0013] Also, some liquid bases (e.g., pH >8) or mixed acids will corrodesilicon, and, therefore, there is also a need to protect the wetted pathfrom such fluids.

[0014] In semiconductor manufacturing processes that etch silicon withfluorine, a mask is often used to cover those portions of the waferwhere etching is not desired. Such masks are made of materials which areunreactive or react very slowly with fluorine. Examples of suchmaterials are SiO₂, Si₃N₄, photoresist, or metal films of aluminum ornickel. However, these masks and the corresponding processes are used toselectively etch silicon and have not been employed to provide long-termprotection of the wetted path of a MEMS device from corrosion byfluorine or other elements. In addition, such methods do not provide ameans for identifying devices with inadequate coverage of the protectivematerial.

[0015] Furthermore, such methods typically entail creating a protectivemetal film of aluminum or nickel by exposing aluminum or nickel layersto ClF₃ gas or F₂ gas, where the fluorine in these gases reacts with themetal to create a film, consisting of a nonvolatile fluorine compound,over the metal. The creation of the film provides a “passivating layer”on the aluminum or nickel. Materials, like aluminum and nickel, withwhich fluorine reacts to create a nonvolatile compound, are known toform these passivating layers. The problem with using ClF₃ or F₂ is thatsuch gases are corrosive and highly toxic, rendering the passivationprocess dangerous, difficult, and expensive. For instance, exposure ofsilicon to ClF₃ can produce extreme heat and may result in catastrophicfailure of the MEMS device and associated equipment.

[0016] Applying materials, such as aluminum, nickel, or other protectivelayers, to the wetted path of a multilayer silicon MEMS devices presentsan additional challenge. Some MEMS devices, such as valve 100, arecomprised of two or more silicon wafers fusion bonded together. Thefusion bonding creates hidden flow passages which are difficult toaccess using conventional deposition or electroplating techniques, and,thus such techniques are not suitable for multi-layer MEMS devices.Atomic layer deposition (“ALD”) processes can more easily reach suchhidden passages, but known, true ALD techniques do not enable materialslike aluminum to be deposited in layers thick enough to adequatelyprotect the silicon.

[0017] The hidden passages in a MEMS device also present a challenge inensuring complete protection of the wetted path. It is very importantthat potential defects in the protective film be screened out prior touse in a hostile environment.

[0018] Therefore, there is a need for a process for depositing,passivating, and testing a fluorine-resistant (and/or base or mixed acidresistant) material in the wetted path of a single or multilayer MEMSdevice that is reliable and complete and preferably employs less toxicand corrosive compounds than ClF₃ or F₂ to achieve the passivatinglayer.

SUMMARY OF THE INVENTION

[0019] According to one embodiment, the wetted path of a MEMS device iscoated with a material capable of being passivated by fluorine. Thedevice is then exposed to a gas that decomposes into active fluorineconstituents either spontaneously or, preferably, when activated by aplasma or other energy source. One example of such a gas is CF₄, anunreactive gas which is easier and safer to work with than reactivegases like ClF₃. The gas will passivate the material and corrode anyexposed silicon. The device is tested in a manner in which anyunacceptable corrosion of the wetted path will cause the device to fail.If the device operates properly, the wetted path is deemed resistant tocorrosion by fluorine.

[0020] As discussed above, many MEMS devices are comprised of two wafersbonded together. In one embodiment, each of the wafers, prior tobonding, is coated at least in part with a material capable of bothbeing passivated by fluorine and forming a eutectic bond with silicon.The wafers are then attached by a eutectic bond between the material andthe silicon before being exposed to CF₄ or other similar gas.

[0021] By applying the selected material prior to bonding, it is morelikely that all parts corresponding to the wetted path, including thehidden channels formed after bonding, will be adequately coated with thematerial. Furthermore, choosing a material that can form a eutectic bondwith silicon means the bond will be achieved at a lower temperature thanthe melting point of the material, thereby ensuring the wafers can beattached without destroying the material. Aluminum and nickel areexamples of the materials which can be applied to passivate fluorine andform a eutectic bond with silicon.

[0022] In an alternate embodiment, an organic bonding compound, insteadof a metal, is deposited in those areas of the wafer(s) corresponding tothe wetted path. The organic compound creates a polymer film over thesilicon in the wetted path (which acts as a barrier against fluorine orother compounds) and, in multilayer applications, it can be used to bondthe wafers together. An example of such an organic bonding compound isbenzocyclobutene (BCB). The steps after application of the organicbonding compound (e.g., exposure to CF₄ or another gas with fluorine)are the same as described above.

[0023] An optional step that can be added to both of the above-describedmethods is to place the processed and bonded wafer structure in a plasmaactivated C₄F₈ gas or similar compound. The step, which preferablyoccurs after the exposure to a fluorine-based gas, provides aprotective, Teflon-like film which acts as a further barrier to attackof the silicon by fluorine compounds.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIGS. 1(a) and (b) illustrate cross-sectional views of a knownMEMS microvalve in the “off” and “on” states, respectively.

[0025]FIG. 2 illustrates a method for producing and testing acorrosion-resistant silicon device in accordance with one embodiment ofthe present invention.

[0026]FIG. 3 illustrates a method for producing and testing acorrosion-resistant silicon device using a protective metal layer inaccordance with one embodiment of the present invention.

[0027]FIG. 4 illustrates a method for producing and testing acorrosion-resistant microvalve in accordance with one embodiment of thepresent invention.

[0028]FIG. 5 illustrates the portions of an upper and lower wafer thatcorrespond to a microvalve.

[0029]FIG. 6 illustrates a method for producing and testing acorrosion-resistant silicon device using an organic bonding compound inaccordance with one embodiment of the present invention.

[0030] FIGS. 7(a) and 7(b) illustrate an example process flow forproducing a corrosion-resistant microvalve in accordance with the methodillustrated in FIG. 4.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0031]FIG. 2 illustrates a method for producing and testing acorrosion-resistant wetted path in a silicon device according to oneembodiment of the present invention. Starting with a silicon wafer(s) inwhich the wetted path has been formed, the protective material isapplied 210 to at least those portions of the silicon wafer(s)corresponding to the wetted path. As described in further detail below,examples of such protective material include (1) a metal, such asaluminum or nickel, that can be passivated by fluorine compounds or (2)an organic compound, such as BCB, that is either resistant to fluorineor can be passivated by fluorine.

[0032] The wafer (or multi-layer wafer structure if applicable) is thenexposed 220 to a gas that decomposes into active fluorine compounds,either spontaneously or when activated by a plasma or other energysource. An advantage of using an unreactive gas that requires an energysource to decompose into fluorine compounds, such as CF₄, is that thesegases usually are safer and easier to work with than those thatdecompose spontaneously. The purpose of the gas exposure is to (1)passivate the deposited material (if applicable, as some organiccompounds may be non-reactive to fluorine without passivation) and (2)cause failure in any silicon region in the wetted path not protected bythe deposited material. The conditions of the gas exposure arepreferably optimized for etching silicon, thereby rendering it probablethat any exposed silicon will be attacked and can be identified throughinspection or testing of the completed wafer. The exposure to the gasshould be long enough to cause failure of a silicon region unprotectedby the passivating material during subsequent inspection or testing.

[0033] Examples of gases that may be used are CF₄, SF₆, NF₃, and ClF₃,but those skilled in the art will appreciate that some of the otherfluorine-based gases, such as certain fluorinated hydrocarbons(especially those with an effective fluorine-to-carbon ratio >2), alsowill decompose into active fluorine compounds that will passivate thedeposited metal and etch any exposed silicon. For reference, theSemiconductor Equipment and Materials International (SEMI) draftdocument 3520 titled “Guidelines for Gas Compatibility with Silicon MEMSDevices” dated Sep. 24, 2002 lists many known fluorinated hydrocarbons,and those skilled in the art will appreciate that some of these willetch silicon and passivate metals like aluminum and nickel. The D. H.Flamm, et al. article “The Design of Plasma Etchants,” Plasma Chemistryand Plasma Processing, Vol. 1(4), 1981; p. 317, also includes theetching properties of select fluorinated hydrocarbons. Both theaforementioned Flamm et al. article and the draft SEMI document 3520 areincorporated by reference as if fully disclosed herein.

[0034] After the wafer is exposed to the gas, it is subsequently tested230 in such a way that any unacceptable corrosion of the wetted pathwould likely cause failure of the device. If the wetted path is notadequately protected, the fluorine compounds will corrode it during thegas exposure, and the device will not operate properly during the test.If the device operates properly, the wetted path is deemed resistant tofluorine (and possibly other fluids, depending on the type of protectivematerial deposited).

[0035] As stated above, examples of the deposited protective materialinclude (1) a metal that can be passivated by fluorine (e.g., aluminum,nickel) and (2) an organic compound like BCB. The above-described methodis set forth in greater detail below with respect to using a metal or anorganic compound as the protective material.

[0036] 1. Metal Film

[0037]FIG. 3 illustrates a method, according to one embodiment of thepresent invention, for protecting a MEMS device (or other silicondevice) from corrosion by fluorine compounds by applying and passivatinga metal layer in the wetted path. This method is described with respectto a single wafer, but the method is applicable to either single-layeror multilayer-wafer silicon devices. The method is as follows:

[0038] a. Starting with a completely processed wafer (i.e., completeexcept for protection of the wetted path), the wafer is stripped 305 ofany oxide or nitride and cleaned using conventional techniques. A layerof SiO₂ at least 5 nanometers (“nm”) thick then is grown 310 on bothsides the wafer. The maximum thickness of the SiO₂ layer is notnecessarily critical and will depend on whether there are any thicknessrestrictions for the wafer in order for the device being manufactured tooperate properly. However, the oxide layer should not be so thick thatremoving exposed regions of it would damage the metal film created asset forth below.

[0039] b. At least a 50 nm layer of a metal is deposited 320 on thewafers using conventional physical vapor deposition (“PVD”) or otherwell known techniques. As stated above, such metal should be capable ofbeing passivated by fluorine compounds. The metal is deposited on atleast those surfaces of the wafer that need to be protected fromcorrosion by fluorine atoms. The maximum thickness of the depositedmetal layer on each wafer will depend on any thickness restrictions forthe wafers due to device operation requirements.

[0040] Examples of metals which may be used are aluminum or nickel.Aluminum will react with fluorine molecules to create AlF₃ (Al+3/2 F₂→AlF₃). AlF₃ is a non-volatile compound which forms a film over thealuminum, thereby protecting the aluminum (and, therefore, also thesilicon) from reacting completely with fluorine compounds. Nickel alsoreacts with fluorine molecules to form a protective film, but aluminumis preferred to nickel because the nickel film does not adhere tosilicon as well as the aluminum film.

[0041] c. Any additional wafer-level processing required is completed330.

[0042] d. At any time deemed appropriate after depositing the metallayer (including prior to step 330), the wafer structure is exposed 340to a gas that decomposes to active fluorine compounds. As stated above,one example is CF₄ gas in a plasma discharge, but there are otherfluorine-based gases which can passivate the deposited metal and etchexposed silicon.

[0043] During the exposure, the active fluorine compounds react with thedeposited material in the wetted path to create a non-volatile compound,which forms a protective film over the deposited material. The activefluorine compounds also reacts with any exposed silicon not covered bythe protective film. Thus, the select gas, and the conditions of the gasexposure, are such that the gas passivates the deposited metal andetches any exposed silicon.

[0044] e. The wafer structure is tested 350 in such a way that anyunacceptable corrosion of the wetted path likely would cause failure ofthe device. If the wetted path is not adequately protected, the fluorinewill corrode it, and the device will not operate properly during thetest. If the device operates properly, the wetted path is deemed to beresistant to corrosion by fluorine. The wafer structure also may bevisibly inspected for any gaps in the coverage of the depositedmaterial.

[0045] In an alternate embodiment, preferably after step (d) and priorto step (e), the wafer is placed in a plasma activated gas of C₄F₈. Thisprocess creates a polymer, Teflon-like film over the wafer and in thehidden channels, which provides an additional barrier to attack byfluorine compounds. During this step, the wafer temperature ismaintained below 50° C. Other fluorinated hydrocarbons with an effectivefluorine-to-carbon ration ≦2 generally may be used as an alternative toC₄F₈ (for instance, a CHF₃ and Argon mix, which has an effectivefluorine-to-carbon ratio of 2 because the hydrogen atom is treated likea carbon for determining the ratio). For reference, the article datedOct. 16, 2000 and titled “Hydrophobic valves of plasma depositedoctafluorocyclobutane in DRIE channels” by Helene Andersson, Wouter vander Wijngaart, Patrick Griss, Frank Niklaus, and Goran Stemme (the“Andersson et al. article”), the contents of which are incorporated byreference as if fully included herein, describes some of the effects ofdepositing C₄F₈ on silicon.

[0046]FIG. 4 illustrates an application of the method of FIG. 3 to themanufacturing of a multilayer-wafer microvalve, such as the valveillustrated in FIG. 1(a) and 1(b). The microvalve application is merelyan example, and the method described with respect to FIG. 4 can beapplied to other multilayer devices having two or more wafers. Forreference, FIG. 5 illustrates the portions of the silicon membrane andchannel plate wafers that correspond to the microvalve 100 illustratedin FIG. 1(a) and (b). The method is as follows:

[0047] a. Prior to bonding, completed membrane and lower wafers 528, 530are stripped 400 of any oxide or nitride and cleaned 400 usingconventional techniques. A 5-50 nm layer of SiO₂ is then grown 405 onboth sides of the membrane wafer 528. The SiO₂ is preferably no morethan 50 nm thick on the membrane wafer 528 so as not to compromise theflexibility of the diaphragm 544. On both sides of the lower wafer 530(the channel plate), a 5 nm or thicker layer of SiO₂ is grown 410.

[0048] b. Anywhere from 50-300 nm of a suitable metal (i.e., one thatcan be passivated by fluorine, preferably aluminum) is deposited 415 onall parts of the flow side of membrane wafer 528 except for bondingregions, which are protected preferably by a shadow mask. Depositingmore than 300 nm may cause the passivated metal film to peel as thediaphragm 544 moves or may impair movement of the diaphragm 544 itself.

[0049] On the lower wafer 530, 50 nm or more of the metal is deposited420 uniformly on both sides of the wafer. Care is taken to covercompletely all of the vertical channels. The metal may be applied to thewafers using conventional PVD or other well known techniques.

[0050] By applying the metal prior to bonding the silicon waferstogether, all parts corresponding to the wetted path are more likely tobe adequately coated with the material. Since the metal is applied priorto bonding, it is advantageous to select a metal, such as aluminum ornickel, that can form a eutectic bond with silicon. Such a metal allowsthe wafers to be bonded without fusion bonding. Fusion bonding typicallyrequires temperatures greater than 900° C., and the preferred metals forpassivating against fluorine will melt at such temperatures. Eutecticbonding occurs at a temperature lower than the melting point of themetal and the silicon. For instance, aluminum and silicon form aeutectic bond at ˜577° C., which is lower than the melting point ofaluminum (˜660° C.) and the melting point of silicon (1430° C.),enabling the wafers to be bonded without harm to either the aluminum orthe silicon.

[0051] In steps 415 and 420, the purpose of applying metal to thebonding region of the lower wafer, but not the membrane wafer, is toenable the two wafers to be joined by a metal-silicon eutectic bond. Inthis microvalve example, the wafers are eutectically bonded together,but, in general, the wafers may be bonded in other ways, such as by anorganic bonding compound, provided that the bonding process does notdestroy the deposited metal in the wetted path. If the wafers are joinedby a means other than through a eutectic bond, then the metal is notdeposited in the bonding region of either wafer unless required by theselected bonding process.

[0052] c. The exposed SiO₂ in the bonding region of the membrane wafer528 is then removed 425 by conventional wet or dry techniques just priorto the eutectic bonding step. Care is exercised that none or very littleof the aluminum deposited on the membrane wafer 528 is removed.Subsequently, if required, the wafers 528, 530 are cleaned 430 usingconventional techniques.

[0053] d. The two wafers are aligned and mated 435 through a eutecticbonding process. The bonding is done in a non-oxidizing and, preferably,reducing atmosphere (which is typically achieved by adding hydrogen to agas stream to prevent oxidation) in a furnace. Placing the wafers on aflat surface in the furnace with additional weight, such as a one poundquartz disc, on top of them can facilitate the bonding step. Foraluminum, temperatures above 577° C., but below 660° C., are preferredand may be necessary depending on the flatness of the wafers and theweight applied to them.

[0054] Some example processing steps for mating the membrane wafer andthe lower wafer include aligning the wafers; placing them in contact;inserting the wafer pair in the furnace on a flat surface with weight ontop of them; purging the furnace with an inert gas; purging the furnacewith a reducing (without being explosive) gas, such as a forming gas(10% hydrogen in nitrogen); heating the furnace to between 577° C. and650° C. for one minute or longer, and then removing the bonded wafers.

[0055] e. A Pyrex layer (e.g., heater plate 22), which includes theheating unit for the microvalve, is anodically bonded 440 to the bondedwafer structure using conventional techniques. Also, any additionalwafer-level required processing is completed.

[0056] f. Same steps as those described above with respect to step 340in FIG. 3. In one embodiment, the wafer structure is loaded into aplasma reactor and exposed 445 to CF₄ at two torr for one hour at 350watts.

[0057] g. The microvalves in the wafer structure are tested 450 toensure proper operation. In one embodiment, the testing occurs after thewafer has been diced into individual valves. If the wetted path is notprotected properly and completely by a protective film, then thefluorine compounds will have corroded the silicon in the wetted pathduring the fluorine-based gas exposure. As the diaphragm 544 is thin, itwill break during operation if it is significantly corroded. In oneembodiment, a valve die is tested by extending the membranepneumatically to 200 psig or more. Any microvalve operating properlyduring testing is assumed to have resisted corrosion by fluorinecomponents. In one embodiment, the valves are also visibly inspected forany gaps in the coverage of the protective material.

[0058] For reference, FIGS. 7(a) and (b) illustrate a more detailedexample of a process flow, at the wafer and die level, for creating amicrovalve in accordance with the embodiment illustrated in FIG. 4. Thesteps from the section titled “Eutectic Bond Process Flow” and belowrelate to protection of the wetted path. The steps in the sections“Upper Silicon Process” and “Lower Silicon Process” relate to etching ofthe wafers and not to protection of the wetted path.

[0059] 2. Organic Bonding Compound

[0060] In an alternate embodiment, the silicon-formed wetted path isprotected from a corrosive fluid by applying an organic bonding compoundto the wafers prior to bonding. The compound creates a polymeric filmover the silicon in the wetted path and, in multilayer applications,bonds the wafers together. An example of such an organic bondingcompound is benzocyclobutene (BCB).

[0061] An organic bonding compound can be used to protect against fluidsother than fluorine. Liquid environments of pH>8 and mixed acids willcorrode silicon, and, unlike the metal films, compounds like BCB willprotect the wetted path against such acids and bases, although suchorganic bonding compounds often are not as resistant to attack byfluorine as the metal films.

[0062] As illustrated in FIG. 6, an organic bonding compound, such asBCB, is applied 610 to at least the areas of the silicon wafercorresponding to the wetted path. In multilayer applications, theorganic bonding compound may be applied to all the wafers, including thebonding regions. For example, in the microvalve application, the organicbonding compound is deposited evenly on the flow side of the membranewafer and both sides of the lower wafer, with care taken to ensure thatthe vertical channels in the lower wafer are completely covered. Theorganic bonding compound is applied as thin as can easily be donewithout compromising coverage or the ability to form a hermetic sealbetween the wafers. In one embodiment, the thickness of the organiccompound is about 2 microns.

[0063] Methods for applying an organic bonding compound to a siliconwafer are known to those skilled in the art. For reference, one suchmethod is described in the article “Void-Free Full Wafer AdhesiveBonding,” presented at the 13^(th) IEEE Conference onMicroelectromechanical Systems in Miyazaci, Japan, Jan. 23-27, 2000, pp.247-252, by Frank Niklaus, Peter Enoksson, Edvard Kalveston, and GoranStemme, the contents of which are hereby incorporated by reference as ifthe contents were fully disclosed herein. The Dow Chemical Company alsohas made publicly available methods for applying its BCB compound“CYCLOTENE,” which is one type of BCB that may be used.

[0064] In the multilayer applications, the wafers may be bonded 620together with the organic bonding compound. When additional hermeticityis required, the organic bonding material is circumscribed by a hermeticepoxy compound in accordance with the teaching of U.S. Pat. No.6,325,886 B1, the contents of which are incorporated by reference as iffully disclosed herein.

[0065] The wafer structure is subsequently exposed 630 to a gas thatdecomposes to active fluorine compounds either spontaneously or whenactivated by a plasma or other energy source. As discussed in moredetail above, one example of such a gas is CF₄ (in a plasma discharge),but there are other fluorine-based gases which can be used. Theconditions of the gas exposure are preferably optimized for etchingsilicon, thereby rendering it probable that any exposed silicon will becorroded. In one embodiment, the wafer structure is loaded into a plasmareactor and exposed to CF₄ at two torr for one hour at 350 watts.

[0066] If the organic bonding compound has been properly and completelyapplied to the wafers, it will act as a barrier between the fluorinecompounds and the silicon in the wetted path. Specifically, the organicbonding compound will either be passivated by the fluorine, non-reactivewith the fluorine, or slowly reactive with the fluorine (slow enoughsuch that it will withstand attack from fluorine during the gas exposurein step 630), depending on the particular organic compound used. Toensure complete protection, the wafer structure is tested 640 in such away that significant corrosion of the wetted path would cause failure ofthe device. If the device operates properly, the organic bondingcompound is presumed to completely cover the wetted path. In oneembodiment of the microvalve application, the testing occurs after thewafer has been diced into valves, which are each tested by extending themembrane pneumatically to some appropriate minimum pressure (forexample, 200 psig). The wafer structure may also be visibly inspectedfor any gaps in the coverage of the organic bonding compound.

[0067] If a particular organic compound is merely slowly reactive tofluorine compounds (as opposed to being essentially non-reactive), thensuch organic bonding compound will likely be used to protect the wettedpath against fluids other than fluorine. Nevertheless, theaforementioned process of exposing the wafer(s) to a fluorine-based gasis still applicable (provided the reaction of the organic compound withthe fluorine compounds is slow enough to withstand attack during suchexposure) as a way to determine whether there are any gaps in thecoverage of the organic compound. Step 630, however, is not necessarilylimited to fluorine-based gases in such cases, as the step also can beaccomplished by exposing the wafer to another type of fluid that iscorrosive to silicon but not to the organic bonding compound.

[0068] In an alternate embodiment, preferably after step 630 and priorto the testing step, the wafer structure is placed in a plasma activatedgas of C₄F₈. This process creates a polymer, Teflon-like film over thewafer, which provides an additional barrier to attack by fluorinecompounds. During this step, the wafer temperature is maintained below50° C. As stated above, other fluorinated hydrocarbons with an actual orequivalent fluorine-to-carbon ratio ≦2 generally may be used as analternative to C₄F₈ (for instance, a CHF₃ and Argon mix).

[0069] The above-described methods have been set forth with respect toMEMS devices, but those skilled in the art will appreciate that suchmethods can also be applied to other silicon devices. Furthermore, aswill be understood by those familiar with the art, the invention may beembodied in other specific forms without departing from the spirit oressential characteristics thereof Accordingly, the disclosure of thepresent invention is intended to be illustrative and not limiting of theinvention.

What is claimed is:
 1. A method for protecting a wetted path of asilicon device from corrosion by fluorine atoms and for testing theadequacy of the protection, the method comprising: coating the wettedpath with a material that can be passivated by fluorine atoms; exposingthe wetted path to a gas environment, which includes fluorine atoms,that will passivate the material and corrode any exposed silicon in thewetted path; testing the silicon device in a manner where corrosion inthe wetted path is likely to cause failure of the silicon device; and inresponse to the silicon device operating satisfactorily during suchtesting, determining that the wetted path is protected from corrosion byfluorine atoms.
 2. The method of claim 1 wherein the material is onethat can form a eutectic bond with silicon.
 3. The method of claim 1wherein the material is aluminum.
 4. The method of claim 1 wherein thematerial is nickel.
 5. The method of claim 1 wherein the silicon deviceis comprised of at least a portion of two wafers, at least one of whichis silicon, bonded together and wherein regions of the waferscorresponding to the wetted path are coated with the material prior tobonding the wafers.
 6. The method of claim 5 wherein the material is onethat can form a eutectic bond with silicon.
 7. The method of claim 6wherein a bonding region of one of the wafers also is coated with thematerial and the two wafers are bonded together through a eutectic bond.8. The method of claim 7 wherein the material is aluminum.
 9. The methodof claim 7 wherein the material is nickel.
 10. The method of claim 1,wherein the gas environment includes a fluorinated hydrocarbon with aneffective fluorine-to-carbon ratio >2.
 11. The method of claim 1 whereinthe gas environment is CF₄ activated by a plasma discharge.
 12. Themethod of claim 1, wherein, after the material has been passivated, thewetted path is exposed to a fluorinated hydrocarbon gas with aneffective fluorine-to-carbon ratio ≦2 to further protect the wetted pathfrom corrosion by the fluid.
 13. The method of claim 12, wherein thefluorinated hydrocarbon gas is C₄F₈.
 14. A method for protecting awetted path of a silicon device from corrosion by a fluid and fortesting the adequacy of the protection, the method comprising: coatingthe wetted path with an organic compound that protects silicon fromcorrosion by the fluid and that is non-reactive or slowly reactive tofluorine; exposing the wetted path to a gas environment, which includesfluorine atoms, that will corrode any exposed silicon in the wettedpath; testing the silicon device in a manner where corrosion of thewetted path is likely to cause failure of the silicon device; and inresponse to the silicon device operating satisfactorily during suchtesting, determining that the wetted path is protected from corrosion byfluorine atoms.
 15. The method of claim 14 wherein the organic compoundis benzocyclobutene.
 16. The method of claim 14 wherein the silicondevice is comprised of at least two silicon wafers and the two wafersare bonded together with the organic compound.
 17. The method of claim16 wherein the organic compound is benzocyclobutene.
 18. The method ofclaim 14 wherein the gas environment is CF₄ activated by a plasmadischarge.
 19. The method of claim 14, wherein, after the wetted path iscoated with the organic compound, the wetted path is exposed to afluorinated hydrocarbon gas with an effective fluorine-to-carbon ratio≦2 to further protect the wetted path from corrosion by the fluid. 20.The method of claim 19, wherein the fluorinated hydrocarbon gas is C₄F₈.21. The method of claim 14, wherein the gas environment includes afluorinated hydrocarbon with an effective fluorine-to-carbon ratio >2.